Create rank depending on 2 variables

[Milena Schmidt]

Hey Stata Community!

I have a question concerning generating a new variable.

My dataset looks like this:

Product	Date
A	05/2017
A	05/2017
A	08/2017
A	08/2017
A	12/2018
A	03/2019
B	01/2014
B	02/2014
C	03/2012
C	03/2012
C	03/2012
C	08/2012
D	01/2017
D	05/2017

What I would like to do is generate a new variable, that tells me the date rank order per product, like this:

Product	Date	Rank
A	05/2017	1
A	05/2017	1
A	08/2017	2
A	08/2017	2
A	12/2018	3
A	03/2019	4
B	01/2014	1
B	02/2014	2
C	03/2012	1
C	03/2012	1
C	03/2012	1
C	08/2012	2
D	01/2017	1
D	05/2017	2

I already tried "egen" and "rank" as commands, but I never got the numbers I wanted in the rank variable.
I think the problem is that I want to create a rank that is based on 2 prior variables (product AND date). (btw: I have my date variable as numeric and as well as string, if I need to take the other one. So far I worked with the numeric one.)

I would be very happy if someone could help!

[Rich Goldstein]

you don't show your code so we can't tell what you did and why it is giving the incorrect results; also, you don't show your data using -dataex- (please read the FAQ) which would make it easy to test new code; but here is a guess

egen rank=rank(Product Date)

if your date variable is not a Stata internal format date, this will not work; see

help datetime

by the way, I have never heard of command called "rank" - where did you find it?

[Milena Schmidt]

Thanks for your response, Rich!

I tried your code:
egen rank=rank(product date)

but unfortunately it says:
"productdate not found" (I don't know why stata assumes this is one variable, when I clearly don't write them together. I tried to seperate them with a comma as well, but then stata says "invalid name".

My date variable is as well coded in a Stata date format.

[Andrew Musau]

Here is another way. Note that Stata variable names are case-sensitive.

bys Product (Date): gen wanted= sum(Date!= Date[_n-1])

[Milena Schmidt]

Thank you so much, Andrew! It worked!